1. Field of the Invention
The present invention relates generally to earth-boring bits of the rolling cutter variety, and more particularly, to the bearing structures used in such bits.
2. Description of the Prior Art
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made the success of rotary drilling possible. Only soft earthen formations could be penetrated commercially with the earlier drag bit, but the two cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the hard cap rock at the Spindletop Field, near Beaumont, Texas with relative ease. That venerable invention, within the first decade of the century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvement of earth boring its of the rolling cutter variety.
In drilling boreholes in earthen formations by the rotary method, earth-boring bits typically employ at least one rolling cone cutter, rotatably mounted thereon. The bit is secured to the lower end of a drill string that is rotated from the surface or by downhole motors. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material. The rolling cutters are provided with teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drill string. As the cutters roll and slide along the bottom of the borehole, the cutters, and the shafts on which they are rotatably mounted, are subjected to large static loads from the weight of the bit, and large transient or shock loads encountered as the cutters roll and slide along the uneven surface of the bottom of the borehole. Thus, most earth boring bits are provided with precision-formed journal bearings and bearing surfaces that are often hardened, such as through carburizing or hard facing, or provided with wear-resistant metal inlays. The bits are also typically provided with seal lubrication systems to increase the drilling life of the bits.
Despite advances in drill bit technology, improvements are still sought to increase the wear-resistance of bearing surfaces to thus increase the life of the drill bit. Super-hard materials, such as natural and synthetic diamond materials, have been used on cutting elements for drill bits for some time. The use of diamond materials for bearing surfaces has had less application, however. Polycrystalline diamond (PCD), for instance, has been used to increase the wear resistance of bearing surfaces in downhole tools. The PCD diamond material is usually formed at high pressure and temperature conditions under which the super-hard material is thermodynamically stable. This technique is conventional and known by those skilled in the art. For example, an insert may be made by forming a refractory metal container or can to the desired shape, and then filling the can with super-hard material powder to which a small amount of metal material (commonly cobalt, nickel, or iron) has been added. This may be capped with a cemented carbide blank or substrate. The container is then sealed to prevent any contamination. Next, the sealed can is surrounded by a pressure transmitting material, which is generally salt, boron nitride, graphite or similar material. This assembly is then loaded into a high-pressure and temperature cell. The design of the cell is dependent upon the type of high-pressure apparatus being used. The cell is compressed until the desired pressure is reached and then heat is supplied via a graphite-tube electric resistance heater. Temperatures in excess of 1350.degree. C. and pressures in excess of 50 kilobars are common. At these conditions, the added metal is molten and acts as a reactive liquid phase to enhance sintering of the super-hard material. After a few minutes, the conditions are reduced to room temperature and pressure. The insert is then broken out of the cell and can be finished to final dimensions through grinding or shaping.
The main problem with these PCD materials is that the diamond formed using this method has limited shapes due to the constraints of the high temperature high pressure (HTHP) apparatus that is used. The PCD diamond used for bearing surfaces is thus formed as inserts that are mounted in holes formed in the bearing shaft. As a result, the PCD diamond may form only a portion of the bearing surface. One example of the use of PCD inserts is described in U.S. Pat. No. 4,738,322. The PCD materials are also very costly because of the small amounts that can be run in a HTHP cell. Use of the binder material also lowers the thermal limits of the insert and can increase the surface friction of the insert.
It therefore would be advantageous to provide a bearing structure for use in an earth-boring bit that has a durable, wear-resistant bearing surface formed of diamond that does not contain binders and can be formed into a variety of different shapes to effectively form a bearing surface.